1. Field of the Invention
The invention concerns the field of ceramics and mechanical engineering and relates to modified silicon nitride composite powders for thermal coating technologies as used, for example, for coating highly stressed components, such as chemical equipment, for example, by means of thermal spraying, and a process for producing such modified silicon nitride composite powders.
2. Discussion of Background
As a result of its favorable chemical, thermal, and mechanical properties, silicon nitride is a material that can be used in many ways (L. Michalowsky, Neue keramische Werkstoffe [New Ceramic Materials], Deutscher Verlay fur Grundstoffindustrie, Leipzig Stuttgart, 1994, p. 395-432). Processing silicon nitride into coatings through various coating technologies, but especially through thermal spraying, has been attempted repeatedly. Sublimation or decomposition hinders processing without additives. In the past, therefore, silicon nitride coatings were attempted to be produced by means of various materials engineering approaches and spraying processes.
In use of composite powders of silicon nitride with metals such as nickel (E. Lugscheider, R. Limbach A. Liden, J. Lodin, High Temp. Mater. Power Eng. 1990, Proc. Conf., Vol. 1, p. 877-880), coatings are indeed produced, but they are comprised of brittle silicides. The properties of these coatings are entirely unsatisfactory for technical applications.
Furthermore, silicon nitride coatings have been attempted to be produced through reactive spraying of metallic silicon in a nitrogen atmosphere onto pre-heated graphite substrates (T. Eckardt, W. Mallener, D. Stover, Proc. 7th National Thermal Spray Conference 1994, ASM International, p. 515-519). However, the reaction rate is so slow that only a very low nitridation is achieved and thus only a non-homogeneous silicon nitride coating can be produced. Only at very high temperatures is a high nitridation achieved on the outer surface of the coating. However, the process is altogether inappropriate for metallic components owing to the high nitridation temperatures used (up to 1,200.degree. C.).
Furthermore, coating powders have been attempted to be produced that comprise larger particles of a non-fusing but decomposing material such as Si.sub.3 N.sub.4 or SiC. These larger particles are surrounded by several small particles of an easily fusing material, such as Al.sub.2 O.sub.3 or spinel (JP, A, 63169371).
In addition to the ratio of sizes of two particles that is unfavorable for thermal coating techniques the non-fusing component is not adequately protected by the easily-fusing particle surrounding it.
EP,A, 0 118 249 discloses a granulated composition that consists of 50-99 parts of a powdered material (powdered metals, temperature-resistant ceramics, cermets, and resins) and of 1 to 50 parts whiskers that act as reinforcing material for the metals, ceramics, and plastics.
The only known successful attempt so far to produce coatings containing silicon nitride with known thermal coating techniques is the use of pre-reacted sialons (Si.sub.6-x Al.sub.x O.sub.x N.sub.8-x where x=3-4) having a high degree of substitution (S. Sodeoka, K. Ueno, Y. Hagiwara, S. Kose, J. Thermal Spray Technology, Vol. 1 (1992), p. 153-159). In this way, coatings containing silicon nitride can be produced that have a porosity of &gt;30% and are thus not dense.
The disadvantage of all prior art processes for producing silicon nitride coatings is that so far, no silicon nitride powders could be produced with which silicon nitride coatings that are usable for technical applications could be produced with known coating techniques.